There is no April security update. The last security update was Mar 21, 2015 and without any further information about your Mac and which version of OS X it's running, it's difficult to try to assist you.
OK, thanks for updating your specs. A security update for Mavericks is different than one for Yosemite. When you say links in mail do not work, are you referring to links that are included with email you receive such as web pages, attachments, and so forth?